This invention relates to an improvement in the synthesis of para-nitrodiphenylamines. Para-nitrodiphenylamines are useful intermediates in the formation of rubber antioxidants and antiozonants. Their generic formula is as follows: ##STR1## wherein R and R.sub.1 are selected from the group consisting of hydrogen radicals and alkyl radicals of 1 to 9 carbon atoms; R.sub.2 and R.sub.3 are selected from the group consisting of hydrogen radicals, alkyl radicals from 1 to 9 carbon atoms, alkoxy radicals of 1 to 9 carbon atoms and cycloalkyl radicals of 5 to 6 carbon atoms.
Presently, these compounds are synthesized by reacting (1) para-halonitrobenzenes conforming to the following structural formula: ##STR2## wherein X is a halogen selected from the group consisting of chlorine and bromine; and wherein R and R.sub.1 are defined above; (2) with a primary aromatic amine of the following structural formula: ##STR3## wherein R.sub.2 and R.sub.3 are defined as above; (3) in the presence of a neutralizing agent, selected from the group consisting of alkali metal salts, oxides of alkali metal salts and alkali metal hydroxides; and (4) in the presence of a catalyst at a concentration of at least 0.1 parts by weight per hundred parts per weight of the para-halonitrobenzene; (5) at a temperature of 120.degree.-250.degree. C.; (6) at a pressure of from atmospheric to about 300 kPa (kilopascals) and (7) with an excess of primary aromatic amine of from 3 to 300 percent.
Presently commerically accepted processes for the production of para-nitrodiphenylamines are described in British Pat. Nos. 798,148; 834,510; German Pat. Nos. 185,663 and 1,056,619; and U.S. Pat. No. 3,155,727. U.S. Pat. No. 4,435,599, by the inventor herein, describes a co-catalyst system for the above described reaction.
Polish Pat. No. 101,496 discloses a method for producing para-nitrodiphenylamines from aniline and para-chloronitrobenzene in the presence of an acid acceptor with concomitant azeotropic water removal using cupric-oxide and dimethylformamide or copper-metal as the catalyst, the improvement being the addition of zinc dust in an amount no larger than 2 percent relative to para-chloronitrobenzene. This Polish patent does not suggest or disclose the use of zinc (II) compounds, (zinc in the plus two oxidation or valence state) to achieve a substantial reduction in the reaction times and provide for increased yield with fewer side reactions.
The presently accepted routes to para-dinitrodiphenylamines (PDNDP) using a p-halonitrobenzene intermediate are the copper catalyzed reactions and the formanilide process. These two routes are different in that the Cu catalyzed reactions require long reaction times (10-24 hours), provide moderate yields of product (75-90%) with less pollution in the waste water than the heretofore mentioned formanilide process.
In the formanilide process, one first reacts formic acid with the primary aromatic amine in the presence of an azeotroping solvent such as toluene. The water of the reaction is azeotroped off at pot temperatures of 125.degree. C. or less. The formanilide produced (such as phenylformamide, C.sub.6 H.sub.5 NHCHO) is then reacted with a p-halonitrobenzene in the presence of a neutralizing agent to yield the p-nitrodiphenylamine, with reaction times of 8-10 hours. A 50% excess of formanilide is required to achieve good yields; however, excessively high COD's in the waste water results.
U.S. Pat. No. 4,155,936 by the present applicant is herein incorporated by reference and made a part hereof. Specifically, U.S. Pat. No. 4,155,936 is concerned with the incorporation of solubilizing agents in the reaction mixture to reduce reaction times and improve yields.
U.S. Pat. No. 4,155,936 and other publications do not suggest or disclose the use of zinc (II) compounds in the preparation of para-nitrodiphenylamines from para-halonitrobenzenes and primary aromatic amines via the formanilide process.
The present invention provides shorter reaction times, lower levels of excess formanilide and neutralizing agent with lower levels of environmentally unsound effluents from the reaction than presently known in the art.
The patents and literature cited do not suggest or disclose the use of any metal catalyst in the formanilide process and the unexpected improvements in the synthesis of para-nitrodiphenylamines that can be obtained.